Electronic, non-invasive techniques for determination of oxygen content are known. U.S. Pat. No. 2,706,927 to Wood disclosed the computation of oxygen saturation from measurement of light absorption of body tissue at two wavelengths. A "bloodless" measurement was first taken in which as much blood as possible was squeezed from the area where measurement was taken. Thereafter, arterial blood was allowed to flow into the tissue as the condition of normal blood flow was restored. A comparison of the light absorption in the two states provided information on the arterial oxygen saturation of the subject. A series of devices and procedures have been founded using this technology.
In procedures based on this technology, difficulty has been experienced in reliably determining the "bloodless" parameters; due in part to geometrical distortion due to the compression of the tissue; imperfect measurement of this parameter gave imperfect results.
The transmission of light of each wavelength is a function of the thickness, color, and structure of skin, flesh, bone, blood and other material through which the light passes. This attenuation in transmission has been asserted to have a logarithmic characteristic, in accordance with the Lambert-Beers Law.
In a pulse oximeter, the primary material of interest is pulsatile arterial blood. Arterial blood is the only material whose quantity in the tissue varies with time in synchrony with the beating of the heart. Variations in light transmission therefore indicate variations in blood flow permitting direct optical recording of the pulsatile component of arterial blood flow. This ability to separate out the light absorption of arterial blood is especially convenient; since the oxyhemoglobin component of blood is a substance for which the absorption coefficients can be determined, the fraction of any oxyhemoglobin in arterial blood can be determined.
Optical plethysmographs are well known. Such instruments measure pulse rate and provide information on the quantity of blood forced into the tissues on each heart beat. These instruments generally utilize a light frequency near or at the isobestic point where measurement of pulsatile flow is made independent of oxygen saturation. Consequently, they intentionally eliminate information on oxygen saturation.
Following the Wood U.S. Pat. No. 2,706,927 patent, numerous attempts have been directed at eliminating the difficulties connected with arterial saturation measurements using light absorption where the analysis requires the comparing the "bloodless" measurement either artificially induced or naturally occuring during the rest state of the heart cycle with the measurement of fresh arterial blood when fresh arterial blood enters the tissue. For example, the signal received has been divided into its "AC" and "DC" components and passed through a log amplifier before digital analysis of the signal occurs. See Koneshi et al., U.S. Pat. No. 3,998,550. Likewise, a generation at both wavelengths of subtraction outputs has been utilized before digital analysis. Subtraction outputs have been used to eliminate the DC component and to approximate the logarithmic response of the prior art. See Hamaguri, U.S. Pat. No. 4,266,554. Simply stated, because the pulsatile component constitutes a small portion of the total signal of transmitted light, numerous manipulations based on logarithms have been attempted to screen out the unchanging component of the resultant signal before analysis.
U.S. Pat. No. 3,704,706 to Herczfeld et al disclosed use of a single coherent red light source, preferably a laser. Use of a single light source is unable to separate information dealing with the arterial flow component from that dealing with the arterial oxygen component. The output of such a single red light source instrument can only be an indication of the product of blood flow and the saturation level present. Neither blood flow alone or saturation alone can be known.
In all of the above schemes for the measurement of pulse rate, pulse flow and oxygen saturation, the variant or AC component is a small portion of the total absorption occurring. In such circumstances, discrimination of the signal from other possible sources must occur. When it is remembered that measurements of unconscious, partially anesthetized and otherwise non-responsive patients must occur, and such patients have random and irregular movements (and heart beats), the establishment of thresholds for the reception and analysis of data is critical.